add ipv6 to xnet.StackAsync (#107)

* add ipv6 to xnet.StackAsync

* dns improvements

* improve DNS workings of StackAsync

* add tentative ICMPv6

* work on prefixes and fix some small bugs, plan UDP/TCP6

* fix bugs in StackAsync and ipv4.Prefix.Contains

* update arpsubtable

* completely remove legacy internet.StackIP for StackIPv4/v6

* ipv4/ipv6 tcp/udp

* add TCP6/UDP6 dialing APIs

* add xnet.Stack6 interface

* more ipv6 integration into StackAsync; various tweaks to lneto and documentation+TODOs

* add stack6 tests

* replace netip.Prefix with ipv4.Prefix where it makes sense

examples/xcurl/main.go

@@ -159,7 +159,7 @@ func run() (err error) {
 			pfbuf, err = pf.FormatFrames(pfbuf, frames, pkt)
 			addr := stack.Addr4()
 			pfbuf = bytes.ReplaceAll(pfbuf, addr[:], []byte("us"))
-			pfbuf = bytes.ReplaceAll(pfbuf, ethernet.AppendAddr(nil, stack.HardwareAddress()), []byte("us"))
+			pfbuf = bytes.ReplaceAll(pfbuf, ethernet.AppendAddr(nil, stack.HardwareAddr()), []byte("us"))
 			pfbuf = append(pfbuf, ']', '\n')
 			if err != nil {
 				return err
@@ -248,7 +248,7 @@ func run() (err error) {
 		return fmt.Errorf("ARP resolution of router failed: %w", err)
 	}
 	timeResolveRouterHW()
-	stack.SetGateway6(routerHw)
+	stack.SetGatewayHardwareAddr(routerHw)
 	if flagDoNTP {
 		timeLookupNTP := timer("NTP IP lookup")
 		const ntpHost = "pool.ntp.org"